A fuel cell is an electrochemical generator that converts chemical energy into electrical energy and that, similar to internal combustion generators, may not require recharging, but rather fuel. A Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a particular type of fuel cell that is well suited for portable applications and which has therefore been quickly developed over the last few years.
One such cell type may advantageously be capable of working in a satisfactory manner even at low temperatures, and moreover may have a good capacity for cold starting in short times. Fulfilling such characteristics may involve the use of catalysts capable of activating, at low temperatures, the chemical reactions which involve the fuel, and which may be constructed from expensive materials, for example, platinum.
The catalyst has primary importance in the functioning of the fuel cell. In particular, the greater the exploitation of the catalyst, the greater the performance of the fuel cell in terms of quantity of current supplied and in the decrease of related costs.
A good catalyst may have a high dispersion coefficient, which is defined as the ratio between the number of surface atoms and the number of total atoms of the catalyst. The higher the ratio, the more efficiently the catalyst is exploited, since the available surface area for the reactions is greater.
In the case of a fuel cell, however, the situation is complicated due to the fact that there are additional factors which affect the catalyst utilization efficiency, as will be explained below with reference to the structure and functioning of a Polymer Electrolyte Membrane Fuel Cell or PEMFC. In detail, the functioning of a PEMFC is essentially assured by two electrodes, anode and cathode, where the electrochemical reactions occur that generate electricity, and by an electrolyte with the function of transporting ions (protons) from the anode to the cathode, which in this specific case may be a polymer membrane.
The fuel is generally hydrogen or methanol which by chemically reacting produces electricity and, as reaction products, water and possibly carbon dioxide depending on the fuel used. The reactions which take place at the electrodes, in the case of hydrogen fuel cells, include:H2→2H++2e− (anode)1/2O2+2e−+2H+→H2O (cathode)
while in the case of direct methanol fuel cells, of:CH3OH+H2O→6e−+6H++CO2 (anode)3/2O2+6e−+6H+→3H2O (cathode)
The assembly comprising the electrodes and the electrolyte membrane—MEA (Membrane Electrode Assembly)—for a cell of the above type is schematically represented in FIG. 1. The (polyelectrolyte) polymer membrane is arranged between the two electrodes and has the dual function of electrically isolating the anode and cathode, and of making the protons, developed at the anode, pass through it, so that the electrons can be provided to an external load to then be used up together with the protons once they have reached the cathode.
The two electrodes in turn comprise a catalytic layer in direct contact with the membrane and a diffusive layer. In brief, the diffusive layer supports the catalytic layer and acts as collector of the electrons developed in the latter and diffuser of the chemical reagents, as can be noted with reference to FIG. 1. The diffusive layer may be hydrophobic to avoid the absorption of water by the polymer membrane and, specifically by the cathode, to favor the flow of the water produced during the respective electrochemical reaction. Therefore, to acquire hydrophobic properties, the diffusive layer is generally treated with Teflon.
Alternatively, it is also possible to introduce a hydrophobic and conductive layer, typically including Teflon and Carbon Black (CB), between the catalytic layer and the diffusive layer, as illustrated in FIG. 2, in which a graphite diffusive layer is represented. The catalytic layer, on the other hand, contains the catalyst that generally is in the form of particles supported by an electrically conductive filler.
In particular, the catalyst may be made of different metals than the two electrodes, which preferably comprise platinum or platinum alloys with cobalt or chromium for the cathode electrode, and of ruthenium, rhodium, iridium, palladium, platinum and their alloys for the anode electrode. The electrically conductive filler generally includes a Carbon Black filler and forms an electronic transport phase or electrically conductive phase to provide the catalytic layer with the capacity to conduct electrons involved in the electrochemical reactions.
The catalytic layer also comprises a proton transport phase (electrolyte phase) which allows the protons generated at the anode to flow toward the electrolyte membrane, and then to the cathode. Typically, the proton transport phase comprises the same material as the electrolyte membrane, to favor the integration with the membrane itself.
Moreover, the catalytic layer may hopefully be equipped with a certain porosity, such that the flow of fuel to the anode and oxygen to the cathode is favored. Overall, the porosity forms that which is defined as the porous phase. In other words, the catalytic layer may favor the transport of the reagents, which is more effective the more it occurs through the porous phase, the (ionic) proton transport through the protonically conductive phase and the electronic transport via the electron conductive phase.
Therefore, the efficiency of the fuel cell is related to the interaction that the phases have with the catalyst and to the continuity which these have inside the catalytic layer. Substantially, then, the more there is simultaneous contact, in the catalytic layer of an electrode, between the porous phase, the protonically conductive phase and the electrically conductive phase supporting the catalyst, the more the cell works efficiently. To form the catalytic layer, the prior art provides several processes, briefly described below.
A first process provides the making of a catalyst ink obtained by mixing specific quantities of conductive filler supporting the catalyst, of solvent and of polymer forming the membrane. The ink is then transferred by painting techniques or by spray onto a Teflon or other material support capable of releasing the ink itself in the form of a solid ink layer.
Such solid ink layer is then applied, that is, deposited on, the electrolyte membrane or the diffusive layer by hot pressing. Moreover, in the mixing, one or more agents adapted to favor the formation of the porosity (pore-forming agents) are generally added, which are removed during or after the deposition of the catalyst ink layer, by means of a heat or washing treatment. As pore-forming agent, a soluble and/or removable polymer for heat treatment can be used, for example polysaccharides and polyethylene glycols, or a salt such as a carbonate, being also removable by washing.
One drawback of this process is the fact that the application, by means of hot pressing, of the catalytic layer (solid ink layer) to the electrolyte membrane or to the diffusive layer may involve an undesired decrease of the porous phase, in particular in the case of pore-forming agents removed during the application of the catalytic layer itself. The heat applied in the process in fact leads to a partial melting of the polymer portion of the catalytic layer, which is compacted by the pressure applied with the increase of the overall density.
The increase of the density occurs to the detriment of the channels useful for the diffusion of the reagents, that is, the pores, which are reduced. It should be noted that such drawback is confined to the porous phase, i.e. to the diffusion of the reagents, while the densification is desirable with regard to the proton transport phase, since it favors the adhesion of the catalytic layer itself to the polyelectrolyte membrane.
In case of pore-forming agents removed after the deposition of the catalytic layer, the porous phase obtained, even if improved with respect to that described above, may not be optimal since there is a random distribution of the pores inside the catalytic layer itself. The random distribution of the pores may not ensure a sufficient contact, in terms of cell performance, among the three transport phases, respectively of reagent, electron and proton transport phases with the catalyst particles. Moreover, it should be added that the treatments for removing the pore-forming agents can damage the catalyst.
To overcome the drawback related to the densification, other catalytic layer application techniques were developed, such as for example molding, painting, and spray, which more greatly preserve the porous phase and which thus may not adversely affect the diffusion of the reagents. These techniques, however, may lead to a reduction of the interaction between the catalytic layer and the polymer electrolyte membrane, with a consequent decrease of the proton exchange.
Overall, therefore, the production processes of a catalytic layer for electrodes of a fuel cell described above may have the effect of favoring the transport of the reagents over the proton transport, or vice versa, depending on the heat and the pressure applied, involving an undesired limitation of the efficiency of the catalytic layer itself.
That set forth above is reported in the example of FIG. 3, where a limited use of the catalyst is illustrated, which results effective near the surface of the catalytic layer. Moreover, it should be observed that the more the thickness of the catalytic layer is increased for increasing the catalyst filler and thus the electrode performances, the more the issue occurs.
To overcome the above drawback related to the excessive or insufficient densification of the catalytic layer, the prior art has developed an embodiment variant of the above-described process, in which a “multiple” catalytic layer is made through the overlapping of several catalyst ink layers applied with different pressures and which therefore form different layers with individual densities.
In practice, after the application of a first ink layer, which forms a first denser portion of the multiple catalytic layer to favor its interaction with the electrolyte layer, subsequent individual layers of catalyst ink are applied, hot but with decreasing pressures. In this manner, the individual layers have increasing porosity as one moves away from the electrolyte membrane and moves closer to the diffusive layer, and consequently the transport of the reagents towards the catalyst particles is favored in such layers.
Generally, then, the last layer may be applied by means of painting, spray or other so-called “pressure free” techniques, that is, without exerting pressure in the application operation of the individual catalyst ink layer. Even if advantageous, such an embodiment may still have drawbacks, the main one of which is the fact that the contact between the three phases, that is, the contact between porous phase, protonically conductive (electrolyte) phase, and electrically conductive phase supporting the catalyst, may be merely partially improved.
In substance, the porous and proton transport phases may not have a corresponding progression along the multiple catalytic layer, i.e. they may not be improved in parallel. In other words, along the thickness of the multiple catalytic layer, the contact between the proton transport phase and the porous phase may not be uniformly improved since, if one considers the different density portions of the multiple catalyst layer, one obtains a diminution of the proton transport capacity with an increase of the porous phase, and vice versa.
It should moreover be added that the integration between the catalytic layer and the electrolyte membrane can also be improved through the use of a binder of known type, such as for example polyethylene, polypropylene, polycarbonate, polyamide and similar binding agents. In such case, the binder is initially added to the starting mixture which forms the catalyst ink. The binders are generally used when the polymer forming the electrolyte membrane may not be soluble in the solvent used in the starting mixture. It should be observed that one such expedient produces an improvement in the contact between the proton transport phase and the electron transport phase.
According to further embodiments of the above-described process, the contact between the proton transport phase and the electron transport phase can also be improved through surface treatments of the Carbon Black, which as reported above is generally used as electrically conductive filler supporting the catalyst. Such treatments are directed to making the Carbon Black more hydrophilic. A poorly hydrophilic conductive filler, in fact, may be disadvantageous both in terms of the fuel access and the transfer of the protons toward the polyelectrolyte phase.
Such treatments provide the modification of the surface polarity through the use, for example, of silica, of polar organic groups or by conductor polymers having heteroatoms along the chain such as nitrogen (N), oxygen (O) or sulphur (S). In the latter case, the overall electronic conductivity of the catalytic layer may also be improved, but the overall improvement regards the contact of the catalyst with the electronic transport phase and with the proton transport phase, since the porosity inside the catalytic layer remains randomly distributed. Moreover, the long radius continuity of the individual phases, that is, the uniformity of each phase inside the catalytic layer, may not be ensured, and hence an effective transport of the reagents, protons and electrons may not be ensured.
In a further process provided by the prior art, the production of a catalytic layer of an electrode in a fuel cell provides a so-called “ink-jet” deposition phase which may allow a speeding up of the process of the actual deposition of the catalytic layer with respect to hot pressing techniques. The processes which such ink-jet technique adopts may be carried out continuously and may allow making a pattern with high precision, but they may not resolve the problem related to the efficient use of the catalyst. Such technique, in fact, may have the same drawbacks and limits observed in the case of deposition of the catalytic layer by means of painting or spray.
In other words, even with the use of the ink-jet technique, a poor interaction may be obtained between the catalytic layer and the polymer membrane, with consequent diminution of the proton exchange. Moreover, even the processes of more recent development, such as electrodeposition and sputter deposition, although advantageous, may be applied in the making of very thin catalytic layers, and therefore regard the making of low power fuel cells.